Fix bug for WalManager with compressed WAL (#10130)

Summary:
RocksDB uses WalManager to manage WAL files. In WalManager::ReadFirstLine(), the assumption is that reading the first record of a valid WAL file will return OK status and set the output sequence to non-zero value.
This assumption has been broken by WAL compression which writes a `kSetCompressionType` record which is not associated with any sequence number.
Consequently, WalManager::GetSortedWalsOfType() will skip these WALs and not return them to caller, e.g. Checkpoint, Backup, causing the operations to fail.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/10130

Test Plan: - Newly Added test

Reviewed By: riversand963

Differential Revision: D36985744

Pulled By: akankshamahajan15

fbshipit-source-id: dfde7b3be68b6a30b75b49479779748eedf29f7f

1 files changed, 14 insertions, 4 deletions

diff --git a/db/wal_manager.cc b/db/wal_manager.cc
index 83a3636fb..ed76905d4 100644
--- a/db/wal_manager.cc
+++ b/db/wal_manager.cc
@@ -507,10 +507,20 @@ Status WalManager::ReadFirstLine(const std::string& fname,
     }
   }
 
-  // ReadRecord might have returned false on EOF, which means that the log file
-  // is empty. Or, a failure may have occurred while processing the first entry.
-  // In any case, return status and set sequence number to 0.
-  *sequence = 0;
+  if (status.ok() && reader.IsCompressedAndEmptyFile()) {
+    // In case of wal_compression, it writes a `kSetCompressionType` record
+    // which is not associated with any sequence number. As result for an empty
+    // file, GetSortedWalsOfType() will skip these WALs causing the operations
+    // to fail.
+    // Therefore, in order to avoid that failure, it sets sequence_number to 1
+    // indicating those WALs should be included.
+    *sequence = 1;
+  } else {
+    // ReadRecord might have returned false on EOF, which means that the log
+    // file is empty. Or, a failure may have occurred while processing the first
+    // entry. In any case, return status and set sequence number to 0.
+    *sequence = 0;
+  }
   return status;
 }